Numerous structures have been proposed to preheat hydrocarbon liquid fuel prior to its delivery to a carburetor for an internal combustion engine. Hot exhaust gases and the hot coolants for the engines have been used to preheat the fuel. When the hydrocarbon fuel is heated, it tends to vaporize with the result that vapors along with liquid fuel is delivered to the carburetor. This makes it difficult to control the temperature of the liquid fuel and will starve the carburetor of liquid fuel. This results in engine inefficiency and stalling due to vapor lock. The prior art fuel preheaters are unable to compensate for temperature variations in the heat exchanger and the formation and extraction of fuel vapors from the heat exchanger so that only heated liquid fuel is supplied to the engine.